Semiconductor lasers are formed of multiple layers of semiconductor materials. The conventional semiconductor diode laser typically includes an n-type layer, a p-type layer and an undoped layered active structure between them such that when the diode is forward biased electrons and holes recombine within the active structure with the resulting emission of light. The layers adjacent to the active structure typically have a lower index of refraction than the active structure and form cladding layers that confine the emitted light to the active structure and sometimes to adjacent layers. Semiconductor lasers may be constructed to be either edge emitting or surface emitting.
A semiconductor laser that emits photons as electrons from within a given energy band cascade down from one energy level to another, rather than emitting photons from the recombination of electrons and holes, has been reported by a group at AT&T Bell Laboratories. See, J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, Science, Vol. 264, pp. 553, et seq., 1994. This device, referred to as a quantum cascade laser (QCL), is the first reported implementation of an intersubband semiconductor laser. The basic light-generation mechanism for this device involves the use of 25 active regions composed of 3 quantum wells each. Injection by resonant tunneling occurs in the energy level (level 3) of the first, narrow quantum well. A radiative transition occurs from level 3, in the first well, to level 2, the upper state of the doublet made by two coupled quantum wells. Quick phonon-assisted relaxation from level 2 to 1 insures that level 2 is depleted so that population inversion between levels 3 and 2 can be maintained. Electrons from level 1 then tunnel through the passive region between active regions, which is designed such that, under bias, it allows such tunneling to act as injection into the next active region.
Lasing for such devices has been reported at 4.6 .μm up to 125 K with threshold-current densities in the 5 to 10 kA/cm2 range. F. Capasso, J. Faist, D. L. Sivco, C. Sirtori, A. L. Hutchinson, S. N. G. Chu, and A. Y. Cho, Conf. Dig. 14th IEEE International Semiconductor Laser Conference, pp. 71-72, Maui, Hi. (Sep. 19-23, 1994). While achieving intersubband lasing in the mid- to far-infrared region, the thresholds were two orders of magnitude higher than “state-of-the-art” practical diode lasers. The reason for the high thresholds is that the transition from level 3 to 2 is primarily nonradiative. The radiative transition, with momentum conservation, has a lifetime, TR, of about 26 ns, mostly due to the fact that it involves tunneling through the barrier between the first and second quantum well. By contrast, the phonon-assisted transition, T32, has a relatively short lifetime, i.e., T324.3 ps. As a result, phonon-assisted transitions were about 6000 times more probable than photon-assisted transitions; that is, the radiative efficiency was 1.6×10−4, which explains the rather high thresholds.
Faist, et al. proceeded to improve their QCL device by making two-well active regions with a vertical transition in the first well, and providing a multi-quantum barrier (MQB) electron reflector/transmitter (mirror). J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, A. L. Hutchinson, and A. Y. Cho, Appl. Phys. Lett., 66, 538, (1995). As a result, the electron confinement to level 3 improved (i.e., the reflection aspect of the MQB mirror suppresses electron escape to the continuum), and threshold current densities, Jth, as low as 1.7 kA/cm2 at 10 K were achieved. However, the basic limitation, low radiative efficiency, was not improved, since phonons still dominate the level 3 to level 2 transition. Using a 2 QW active region with a vertical transition in the first well, Jth values as low as 6 kA/cm2 at 220 K were obtained. J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, A. L. Hutchinson, S. N. G. Chu, and A. Y. Cho, “Continuous wave quantum cascade lasers in the 4-10 μm wavelength region,” SPIE vol. 2682, San Jose, pp. 198-204, 1996. An improved version of the vertical transition design was operated pulsed at 300 K, the first mid-IR laser to operate at room temperature in the 5 μm wavelength regime. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Room temperature mid-infrared quantum cascade lasers,” Electron. Lett., vol. 32 pp. 560-561, 1996. A further improvement consisted of using three QWs such that the lower energy level (of the optical transition) is depopulated by using phonon-assisted transitions to two lower levels. D. Hofsteffer, M. Beck, T. Aellen, J. Faist et al., “Continuous wave operation of a 9.3 μm quantum cascade laser on a Peltier cooler”, Appl. Phys. Lett., vol. 78, pp 1964-1966, 2001. This double-phonon resonance approach has allowed lowering the Jth value to 3-4 kA/cm2 and resulted in the first continuous wave (CW) room-temperature operation of QC lasers (λ=9.1 μm). M. Beck, D. Hofstetter, T. Aellen, J. Faist, U. Oesterle, M. Ilegems, E. Gini, and H. Melchior, “Continuous Wave Operation of a Mid-Infrared Semiconductor Laser at Room Temperature,” Science, vol. 295, pp. 301-305, 2002. However, that was achieved with very low wallplug efficiency, ηp, values (<1%) and highly temperature-sensitive characteristics. Recently, strain-compensated structures have allowed CW operation at room temperature at shorter wavelengths (λ=4.3-6.0 μm), but again with low ηp values (<3%) and highly temperature-sensitive CW characteristics. A. Evans, J. S. Yu, S. Slivken, and M. Razeghi, “Continuous-wave operation at λ˜4.8 μm quantum-cascade lasers at room temperature,” Appl. Phys. Lett., vol. 85, pp. 2166-2168, 2004. This poor performance is directly related to the fact that the rise in the active-region temperature with respect to the heatsink temperature is very high (e.g., 70-80° C.), about an order of magnitude higher than for conventional semiconductor lasers. That is why for most effective all-around heat removal a buried heterostructure is needed since the generated heat can be laterally removed.
Botez et al. have proposed the use of 2-D arrays of quantum boxes for increasing the carrier relaxation time by at least a factor of 20. U.S. Pat. No. 5,953,356. Then. ηp values as high as 24% have been predicted. Chia-Fu Hsu, Jeong-Seok O, Peter Zory and Dan Botez, “Intersubband quantum-box semiconductor lasers,” IEEE J. Selected Topics in Quantum Electronics, vol. 6, pp. 491-503, May/June 2000. However, due to relatively low gain, such devices will provide low powers (˜30 mW) from conventional single-element devices. Although it has not previously been suggested, one possibility for increasing the emitted power, is scaling in the lateral direction via Active-Photonic-Crystal (APC) structures.
Since QCLs may utilize a buried heterostructure design in order to assist with lateral heat removal, it is impractical to incorporated such lasers into APC structures for scaling the power, as it is done for other types of lasers.